Prismatic retroreflective sheets such as cube-corner prismatic retroreflective sheets are used as displaying boards such as traffic signs or outdoor advertising signs, since they have higher reflection luminance and improve the visibility at night more effectively than glass beads type retroreflective sheets.
Now, the structures and functions of the prismatic retroreflective sheets will be explained briefly.
The prismatic retroreflective sheet comprises a retroreflective element having a main surface on which a plurality of prismatic projections are arranged, and a lining film made of a light-transmitting resin, and the retroreflective element and lining film are partly bonded so that minute sealed cells are formed between them. The surfaces of the prismatic projections are confined with having interfaces with an air, and the prisms having the interfaces with the air retroreflect the incident light at the high luminance. Sealing projections, which are partly bonded to the retroreflective element and form the sealed cells, are usually formed by embossing the resin film, which is a precursor of the lining film, from its back surface and protruding the surface side of the resin film.
However, such retroreflective sheets have their limit in improving the visibility at night in places where limited light sources such as head lamps of automobiles are available, for example, road sides having no lights nearby, since they are seen by viewers only when the sheets reflect the light which illuminates the sheet. That is, such sheets do not work sufficiently as the traffic or advertising signs which provide information to pedestrians at night in the above circumstances.
Thus, to solve such problems, several proposals have been made, for example, in JP-A-8-502131 (See, U.S. Pat. Nos. 5,315,491 and 5,300,783) and WO92/14173 (See, U.S. Pat. Nos. 5,243,457 and 5,237,448).
These patent publications disclose self light-emitting retroreflective sheets which comprise a cube-corner prismatic retroreflective element, and an EL device or a phosphorescence layer, and have a relatively thin thickness.
That is, the retroreflective sheets of the following types (a), (b) and (c) are disclosed:
(a) A retroreflective sheet comprising cube-corner prisms, and EL devices having transparent upper lids which are in close contact with prism projections on the outermost surfaces. PA1 (b) A retroreflective sheet comprising cube-corner prisms which are confined in sealed cells under the condition that the surfaces of the prism projections have interfaces with an air, transparent adhesive layers which are filled in the cells in contact with the prism projections, and EL devices adhered to the back surfaces of the adhesive layers in the cells. PA1 (c) A retroreflective sheet comprising cube-corner prisms, and EL devices provided in cells which confine the prism projections under the condition that the surfaces of the projections have interfaces with an air at certain distances from the prism projections.
The retroreflective sheet (a) emits light effectively when the EL device is lighted, but does not retroreflect the incident light since the surfaces of the prism projections have no interfaces with the air.
The retroreflective sheet (b) can emit light effectively in areas where the EL devices are in contact with the cube-comer prisms through the transparent adhesive layers, and it can retroreflect the incident light in areas where no EL device is present. However, the EL devices emit light in parts of the sheet surface having the cube-corner prisms, but do not emit light uniformly over the whole surface of the sheet.
The retroreflective sheet (c) can retroreflect the incident light over the whole surface of the sheet but the EL devices emit light in the parts of the sheet surface.
A retroreflective sheet in which a lining film emitting phosphorescence is provided on the back surface of a prismatic retroreflective element is known and disclosed in, for example, U.S. Pat. No. 5,415,911 and JP-A-7-218708. However, this sheet cannot emit the light as bright as the EL devices do.